A secondary Cisco Unified CME router to act as a backup if the primary router fails, perform the following steps on both the primary and secondary Cisco Unified CME routers.

The secondary router`s running configuration must be identical to that of the primary router.

Phones that use this feature must be configured with the type command, which guarantees that the appropriate phone configuration file will be present.

The FXO line from the PSTN is split using a splitter. From the splitter, one line goes to the primary Cisco Unified CME router, and the other line goes to the secondary Cisco Unified CME router. When a call comes in on the FXO line, it is presented to both the primary and secondary Cisco Unified CME routers. The primary router is configured by default to answer the call immediately. The secondary Cisco Unified CME router is configured to answer the call after three rings.

• Configure the secondary CCME IP address

telephony-service
  ip source-address <cme1-ip> [port <port>] secondary <cme2-ip>

• On primary CCME router FXO port
• incoming alerting ring-only
  • Instructs the FXO ground-start voice port to detect incoming calls by detecting incoming ring signals.
• ring number
  • Default is 1 so don't need to configure

voice-port 2/0
  signal ground-start 
  incoming alerting ring-only

• On backup CCME router FXO port
• incoming alerting ring-only
  • Instructs the FXO ground-start voice port to detect incoming calls by detecting incoming ring signals.

voice-port 2/0
  signal ground-start 
  incoming alerting ring-only
  ring number 3